Automated Wing Drilling System for the A380-GRAWDE

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Copyright © 2003 SAE International
ABSTRACT
On Airbus aircraft, the undercarriage reinforcing is
attached through the lower wing skin using bolts up to 1-
inch in diameter through as much as a 4-inch stack up.
This operation typically takes place in the wing box
assembly jigs. Manual hole drilling for these bolts has
traditionally required massive drill templates and large
positive feed drill motors. In spite of these large tools,
the holes must be drilled in multiple steps to reduce the
thrust loads, which adds process time.
For the new A380, Airbus UK wanted to explore a more
efficient method of drilling these large diameter holes.
Introducing automated drilling equipment, which is
capable of drilling these holes and still allows for the
required manual access within the wing box assembly
jig, was a significant challenge. To remain cost
effective, the equipment must be flexible and mobile,
allowing it to be used on multiple assemblies.
In conjunction with Airbus UK, Electroimpact has
developed a mobile automated drilling system for the
A380 undercarriage area. The system can drill up to
1.25-inch diameter holes in one shot. Similar in design
to a five-axis post mill, the system can be transported to
multiple work zones using an adapted stacking crane.
2003-01-2940
Automated Wing Drilling System for the A380-GRAWDE
Scott Hogan, John Hartmann, Brent Thayer and Jack Brown
Electroimpact Inc, USA
Ian Moore, Jim Rowe and Mark Burrows
Airbus UK
The system has been termed the GRAWDE for Gear Rib
Automated Wing Drilling Equipment.
INTRODUCTION
The GRAWDE is a wing-drilling machine specifically
designed for operations in the gear rib area of the A380
wing. The machine drills fastener holes for the purpose
of fastening the gear rib reinforcing through the skin into
the rib spar structure matrix. Fastener holes in this area
of the wing are as large as 1 inch in diameter and 4
inches deep. Manually cutting these large holes is a
multi-step process using numerous pneumatic drill
motors and drill templates. In the assembly jig, this area
of the wing extends just below factory floor level. High
fastener density, numerous drill motors and templates,
and restricted worker access to the reinforcing area for
the workers creates a rate-limiting step in the wing
assembly process. The GRAWDE was designed to
address these challenges.
The key design features of the GRAWDE are:
1. Reduce labor and decrease production time by
replacing the multi step manual drilling process
with single step CNC drilling process.
2. Increase hole quality and significantly decrease
rework with automated drilling.
3. Maximize machine utilization by providing
functionality to move machine into multiple
parallel workzones.
4. Machine fits into an envelope, which allows
manual work access with or without the machine
in place.
5. Adaptive feedback provides real time
countersink depth compensation on faceted
surfaces.
MANUAL PROCESS
In the wing assembly jig, the A380 wing is fixtured
trailing edge down with the gear rib area extending
below factory floor level. A permanent reinforcing plate
is fastened over the lower wing skin and extends from
the aft tip of the wing box up to the rear spar. The 2
meter wide reinforcing plate provide structural support to
the main landing gear. The surface area of this region of
the wing amounts to approximately four squares meters
and contains approximately 250 fasteners; the majority
over 0.75 inches in diameter.
DRILL TEMPLATES
Figure 1 - Drill jig on A340 gear rib area
To achieve proper hole placement using manual
processes, drill jigs like the one shown in Figure 1 are
used. Each manual drill template consists of a
machined aluminum plate with various drill and fixture
bushings. The templates are located and fastened to
the wing skin through backdrilled holes. Due to the area,
size of fasteners, and size of the drill motors, the drill
templates are large, complex and labor intensive to use.
Figure 2 - Pneumatic drill and template
Fastener pitches are too close for the required drill
bushing diameters, which results in multiple templates
for a given area due to the overlap. These templates
are sometimes referred to as half templates as they are
used in pairs to place alternating holes.
The manual drilling technology is not suited for
accurately and efficiently cutting large holes in a single
pass. One hole may require as many as five tool
changes. The hole is started with a small pilot and then
stepped up to the final size. The hole is finished with a
final ream and countersink pass. Table 1, below, shows
a sample drill sequence for a .871-inch diameter hole.
Each drill size uses a dedicated drill motor, which
represents a significant cost element. As shown in the
table below, several types of drill motors may be used
for a single hole. Figure 2 shows a sample drill motor
and template. Drilling these large holes manually can be
a time consuming process and may sometimes require
rework.
Table 1 Example drill step sequence
Hole - .871 Stack - 2.6
Step Tool Description Machine Type
1 DRILL - NON / PILOTED Positive Feed
2 CORE DRILL - NON / PILOTED Positive Feed
3 REAMER - NON / PILOTED Positive Feed
4 BACK SPOTFACE CUTTER RACKFEED
4 SPOTFACE ARBOR
5 COUNTERSINK - FORWARD / PILOTED RACKFEED
6 COUNTERSINK - FORWARD / PILOTED Pistol
6 ARBOR - COUNTERSINK CUTTER
7 GAUGE, PLUG
WORKER ACCESS
The gear rib area has limited access in the assembly jig.
It is located on the first floor with the lowest part of the
trailing edge below factory floor level. From an
ergonomics perspective, this requires manually lifting jig
plates and drill motors into positions, which range from
below floor level up to 2 meters above the ground. The
ideal working range is approximately 1.1 to 1.3 meters
above ground. Work below that range requires workers
to be on their hands and knees and above that requires
an elevated platform.
AUTOMATED PROCESS
The GRAWDE is a CNC controlled five-axis, precision
drilling machine designed to replace the manual drilling
process. The head consists of a single spindle with an
integrated pressure foot. The 22kw spindle spins at up
to 8000 RPM. It uses HSK 80 hydraulic holders
complete with through-bit starvation lubrication. The
spindle provides sufficient stability and power to
accomplish one shot drilling and countersinking of the
large diameter holes present in the A380 gear rib area.
The headstone or pressure foot on the GRAWDE is
fitted with a spherical nosepiece with integral normality
sensors. This permits the GRAWDE to be driven to a
precise location on the wing skin, sense and adjust to
the local normal vector by rotating the drill about the tool
point. The Z-axis drive (parallel the spindle axial feed
axis) is fitted with a force sensing load cell. Allows the
GRAWDE to clamp or press up against wing skin with as
much as 2000 lbs of force for added process stability
and rigidity.
DRILLING
The drilling process starts with CNC programs or tapes.
Airbus programmers develop programs, which define
nearly all aspects of the drilling process. The machine
runs under five-axis CNC control in each the wing
surface zones. The machine operator calibrates the
machine using test coupons, synchs the machine to the
proper zone using a camera target and runs the CNC
tape.
Coupon stands are located in the inboard area of the
assembly jig. Multiple coupons, representative of the
wings skin are used for setup. The coupon stands also
have a re-synch target used for establishing machine
position and calibrating for normality. The re-synch
target consists of a flat plate with a precision hole whose
position and orientation in the coordinate system are
known. Calibrating involves using the re-synch camera
to establish the machine position and then clamping up
to calibrate the normality sensors.
Holes are usually drilled in the coupon prior to drilling the
wing in order to prove the cutting tool and establish
plunge depth for the counter sink. Once the machine is
calibrated, the tape will drive the head to a position easy
for the operator to access the drill head and prompt the
operator to install the appropriate cutting tool. A Balluff
tool identification system is used to verify that the proper
cutter is loaded into the machine. The NC tape drives
the machine to the appropriate position and rotates the
U-axis to the theoretical normal vector. The machine
head then drives along the theoretical normal vector in
force sensing mode.
When the pressure foot engages the wings skin with the
programmed force, the program verifies the actual
normal vector, as measured with real time sensors, is
within a set tolerance of the theoretical vector. If not, the
machine will unclamp, rotate about the tool point, reclamp,
recheck for normality before drilling the hole.
DRILLING FACETED SURFACES
The above example illustrates drilling holes on nonfaceted
surfaces on the wing, i.e. where the fastener’s
axis is normal to the wing surface. The GRAWDE also
has the capability to drill holes on faceted surfaces. For
these areas, the fastener is inserted normal to the wing
skin, but not normal to the exterior surface of the
reinforcing plate.
The rotating clamp nose on the GRAWDE makes it
possible to drill off normal on faceted surfaces with full
clamp load. As mentioned above, the clamp nose is
mounted on a spherical bearing. Because the clamp
nose pivots in a spherical bearing as it contacts the wing
skin, it will rotate normal to the exterior surface while the
drill axis is coincident with the wing skin normal vector.
The integrated normality sensors can be used to verify
that the theoretical facet angle agrees with the actual
facet angle.
INK MARKING
To aid in debugging programs and ensure proper hole
position, the GRAWDE has the functionality to ink mark
the part. This functionality is typically used for part
program try out. This allows the operator replace the
cutting tool with a marking pen and run the drilling
program while inhibiting drill feed. This paints dots on
the wing panel, which can be visually verified for position
prior to actual drilling.
JIG INTEGRATION
One of the greatest challenges of this project was the
integration of a precision CNC controlled drilling
machine into a wing assembly jig, while maintaining
access for manual assembly operations. To meet this
challenge, the GRAWDE was designed concurrently
with the assembly jig. Each jig is a 4 story steel
structure and is about 50 meters long. Manual worker
access is required from both sides of the wing in all
areas for fitting and fastening processes.
JIG CONSIDERATIONS
The jig was designed to provide a stiff structure for
supporting precision CNC machines, while maintaining
maximum manual access. Continuous floors are
required on each of four levels to extend to the surface
of the wing. The floors are moveable for part loading
and to provide machine access. The automatic drilling
machines need services such as air, power,
communications, and position feedback, all of which had
to be well integrated into the jig structure.
GRAWDE CONSIDERATIONS
Floor heights dictated by manual access requirements
determined the size of the GRAWDE working envelope.
To be a cost effective investment it is crucial that the
machine be portable and be able to move between
multiple wing surface zones. The tight working envelope
represents a challenge to provide a safe working
environment due to potential obstacles and tight
clearances between the machine parts and the jig,
creating shear points.
ACCESS
When operating the GRAWDE, the operators stand on a
moving platform 250 mm below factory floor and 500
mm above the machine beds. From the platform,
operators can reach the wing surface, operator controls
and quill box for changing tools. The platform has
handrails and a gate on two sides, to protect the
operator from falling and to protect from shear points
between the GRAWDE and assembly jig components.
Manual access can also gained by driving the GRAWDE
away from the immediate area and standing directly on
the machine bed. A drawing of the machine and
operator platform is shown below in Figure 3.
Figure 3 - Isometric view of GRAWDE and operator
platform
Integrated into the jig on the ground level are
hydraulically actuated bi-fold floors. When in the open
position, the floors allow access for the GRAWDE.
Because visual access is quite limited in the jig when the
floors are open, the doors are of a bi-fold design, which
provides a line of sight from the GRAWDE to the rest of
the ground floor stage 1 jig. When closed the floors
provide a working surface at factory floor height, which
provides optimal worker access to the rear spar area. A
pre-production image of the GRAWDE in the transfer
area with the bi-fold floors open is shown in Figure 4.
Figure 4 - GRAWDE bed and bi-fold floors
POSITIONING
Drilling, while a major portion of the assembly process,
is only part of building a wing. There is a significant
amount of part loading, fitting and fastening, which must
also take place. To maximize the usage of this asset,
the machine had to be able to be transferred between
jigs and work surfaces. One of the major challenges
presented was the requirement to work on multiple
independent machine bed systems. The GRAWDE
beds are each fitted with linear incremental position
encoders. The encoders are distance coded so that the
machine can be homed without driving to a home switch
somewhere at the end of the bed. To establish an
origin, the GRAWDE is driven to the re-synch target and
centered. An offset value is then entered into the CNC.
The CNC contains a data table of offsets for each bed.
Each X-sled has a bed number coded into the wiring to
so that the CNC knows which offset table to use.
The re-synch target is used to establish datums for both
X and Y-axes. It is also used to compensate the
normality sensors in the nosepiece, for pitch and yaw
rotations.
SOFTWARE COMPENSATION
Measurable deviations exist in the rails, bearings and in
the linear encoder scales. To account for these
deviations, each axis on the GRAWDE is electronically
compensated improve position accuracy. Data tables
for each of the beds are stored in a PC on the
GRAWDE. Compensation data is automatically
transferred to the Faunc 15i CNC via serial link for the
appropriate workzone at startup.
Compensation involves establishing a table of offset
values for an axis as function of the position of the same
or another axis. For example, there is a table of values
for compensating the Y-axis positions at various X-axis
positions. Presumably Y-axis compensation with respect
to the X-axis would be to compensate for deviations in
the X-axis rails or beds. Non-linearity of the encoder
scales is also compensated for in the same way.
A laser tracker or laser interferometer is used to
measure the deviations and establish the offset values.
Since the data is measured at discrete positions, the
Fanuc CNC interpolates between values establish a
smooth compensation curve.
GRAWDE SPECIFICS
The GRAWDE’s working envelope covers the entire
A380 inner rear spar workzone for both upper and lower
surfaces. Presently, the GRAWDE can be situated on
any one of twelve (with unlimited expansion possibilities)
monolithic machine bed systems. These beds are
located in a trench such that the top of the beds is below
factory floor. This allows the beds to be covered for
manual operation when the GRAWDE is not in use.
The beds extend from about the first third of the wing on
the inboard end to the transfer area outside of the jigs.
This transfer area is effectively the working envelope of
a modified stacking crane, which can transport the
GRAWDE between any of the twelve beds.
CLAMP NOSE AND NORMALITY
One of the key features of the GRAWDE is the rotating
clamp nose and integrated normality sensors. When
clamped, the nosepiece must be parallel to the outside
skin surface in order to prevent marking the wing skin,
as well as to measure the distance to the panel for
determining counter sink depth. The gear rib reinforcing
plate represents a challenge because the holes are
drilled normal to the wing surface below, but can be off
parallel from the outside surface of the reinforcing plate
by more than two degrees.
Figure 5 - Cross section of head stone and clamp
nose
The clamp nose on the GRAWDE is mounted on a
spherical bearing to allow off-angle drilling while
maintaining tool point distance and clamping ability.
There are linear pots built into the head stone, which
effectively measure the orientation of the nosepiece.
This design is also a more robust normality system as
the delicate components are buried behind the clamp
nose. Figure 5 shows a drawing of a sample headstone
and clamp nose.
The GRAWDE controls have were designed to take
advantage of the spherical motion of the nosepiece by
making coordinated axis moves. It can rotate the axis of
the drill around the tool point or any point along the drill
axis while maintaining the position of the point in space
within .025 mm.
GRAWDE-TRANSPORTER INTERFACE
Figure 6 - Interface assembly
One of most enabling features of the GRAWDE is
relative ease with which this precision machine can be
moved between parallel workzones. This is
accomplished by the use of a modified version of a
standard stacking crane. To facilitate ease of use, the
stacking crane interface was designed with the following
criteria in mind.
1. Maintain the GRAWDE in vertical attitude despite a
poorly centered load
2. Do not allow the GRAWDE to swing when
transported.
3. Connection must be simple and eliminate rigging such
as straps and hooks
To accomplish this, while minimizing the stresses
imposed by a rigid connection, an interface plate was
suspended by four chains approximately 300mm long.
As long as the CG of the load is between the chains it
will maintain a vertical attitude. In addition, as long as
lateral accelerations are limited the load will maintain its
attitude. A drawing of the transporter side interface plate
is shown in Figure 6. An isometric drawing of the
GRAWDE and transporter is shown below in Figure 7.
Figure 7 - GRAWDE and transporter
The effect of the chains while safely maintaining a
vertical attitude is to effectively change the load from a
long pendulum, to a 300mm pendulum with the same
mass, thereby raising the natural frequency. This
configuration however also creates a torsional
pendulum. This can result in rotational oscillations if
excited by sudden the slewing of the mast. To eliminate
these oscillations, three hydraulic dampers were added
between the chains.
Figure 8 - Transporter receptacle plate on GRAWDE
The interface itself consists of four 80 mm mushroom
shaped plungers attached to the interface plate. The
receptacle on the GRAWDE consists of a plate with four
holes to accept the plungers. On the bottom side of the
receptacle plate, there are two keyhole plates. Each of
these has two keyhole shaped cutouts with a lever
mechanism to move them. The mechanism is actuated
manually from a handle on the outside of the GRAWDE.
Detents on the keyhole plates eliminate any possibility of
the plates sliding while under load.
To aid in aligning the transporter to the GRAWDE a
laser crosshair generator is affixed to the interface plate
on the transporter and a cross hair target on the
GRAWDE. The target is fixed to the GRAWDE at about
eye level and rotated about 40 degrees with respect to
the floor to aid in visibility. This allows both rotational
and translational alignment.
There is minimal communication between the GRAWDE
and the transporter. The interface plate on the
transporter has three proximity and four mechanical
switches. The mechanical switches determine whether
the keyhole plates are in the locked, indeterminant or
unlocked position. The proximity switches on the crane
determine that the GRAWDE is present. This is
important as the transporter is also designed to move
the other equipment.
Because the GRAWDE always faces the wing surface
that it’s drilling, it needs to be rotated approximately 180
degrees between beds. To accomplish this, the stacking
transporter is fitted with slewing capabilities.
THE GRAWDE-BED INTERFACE
Figure 9 - The X-Sled
As previously mentioned, the GRAWDE resembles a
five-axis post mill. To make it portable, a means of
separating it from the precision X-axis bearing rails was
necessary. To accomplish this, each bed is fitted with a
100 mm thick base plate permanently fixed to the bed
via bearing cars – also referred to as the X-sled. The Xsled
is shown in Figure 9. The X-sled is fitted with
various features to precisely locate the GRAWDE as
well as clamp it to the X-sled. The X-axis drive motors
and scale are attached to the X-sled.
Machine Pads
Figure 10 - Machine Pad
There are four pads to locate the GRAWDE in the
vertical direction. See Figure 10. Though three pads
would prevent an over constrained condition, four pads
lend stiffness. The X-sled is fitted with 8 bearing cars,
two at each corner of the sled. The machine pads are
located above each pair of bearing cars on the X-sled.
There is a mating set of pads fixed to the GRAWDE.
Each of these is also hardened and precision ground to
create a mating plane perpendicular to the Y-axis rails.
Perpendicularity is measured with a laser tracker.
In order to prevent swarf and dust from contaminating
the mating surface and effecting alignment, the pads are
fitted with automatic covers. Both the GRAWDE and the
sled have covers, and each has a cam and roller to push
the other open as the interfaces approach. Each cover
is spring-loaded in the closed position.
Lateral and Rotational Alignment
To accomplish constraint in the remaining degrees of
freedom, the X-sled is fitted with three pins and the
GRAWDE with three receptacles. The pins are
precision ground 100 mm case hardened steel. Only
two pins are required to constrain the remaining degrees
of freedom, but geometry and visibility prevent ideal
positioning of the pins. One pin is the master, the other
a rough locator, the third is a precision locator. The
precision locator is shorter than the other two and out of
sight from the operator. The master locator and the
rough locator pins can be seen by the operator through
holes in the tower and, serve as visual alignment guides,
but are too close together to provide a good moment
arm for precise yaw alignment.
The master receptacle on the GRAWDE is designed to
constrain in two directions and fixes the GRAWDE’S
position. The rough locator receptacle constrains in only
one direction to prevent rotation about the master pin
without over-constraining. The fine locator, serves the
same purpose, only with preload.
Figure 11 - Location pin receptacle
The receptacles consist of a 38mm thick plate with either
two or four 2-1/2 inch diameter rollers. On the master,
two of the rollers are on floating axels that are sprung to
the center of the receptacle via belleville washers. The
other two are fixed. On the rough locator, both axels are
fixed. On the fine locator, one axel is fixed while the
other is sprung. When in position, the rollers are loaded
with about 4400 newtons force.
Hooks
The flying portion of the GRAWDE weighs about 7000
kg. While this is theoretically enough to prevent skidding
on the pads, a greater margin was desired. To this end,
the base was fitted with hooks and the GRAWDE with a
mechanism to grab and load the hooks, each with about
26600 N for a total of 10600 N. The hooks consist of
100mm square bar with notches machined into them.
The mechanism consists of rollers situated on a ramp,
driven by an air cylinder.
CONCLUSION
The GRAWDE presents the start of a paradigm change
in wingbox assembly. The mobile machine is fully
integrated into the A380 assembly jig to provide both
automation drilling and unrestricted manual access. The
concurrent design of the wing drilling machines and the
assembly jig enabled the design of a system, which
provides superior quality holes in less time, with less
rework in an area which is typically rate-limiting to the
wing build process.
The GRAWDE can precisely position countersunk
fastener holes up to 1.25 inches in diameter through a 4-
inch stack. It has the ability to re-synch to a feature on
the wing or in the jig, drive to a precise location, adjust
for normality and drill a hole in a single shot. It
eliminates the use of pneumatic drill motors and
templates. It reduces drilling time to less than one
minute per hole, while producing superior quality holes.
The mobility of this machine provides maximizing of a
valuable capital asset over multiple parallel workzones.
ACKNOWLEDGMENTS
We the authors would like to acknowledge the
contributions of Alan Ferguson, Keith Robinson, Chris
Wilson and Andrew Smith of Airbus UK for the support
of this exciting system. We would also like to
acknowledge the many engineers at Electroimpact who
help in bring this machine into reality. These include
John Barry. Russ Devlieg, Ben Hempstead, Brent
Huffer, Fred Stillman, Brent Thayer, Michael Dong, Tony
Gale and Jim Yinger.
CONTACT
Scott Hogan
Project Engineer
Electroimpact, Inc.
scotth@electroimpact.com
(425) 348-8090
John Hartmann
Vice President
Electroimpact, Inc.
johnh@electroimpact.com
(425) 348-8090
DEFINITIONS
Laser Tracker: A very accurate 3 dimensional laser
interferometer.
Stack: Term used to describe the total thickness of
various layers of wing skin, internal flanges and
reinforcing plates sandwiched together
Swarf: Coolant, chips and residue resulting from cutting
operations
Tapes: Fanuc controllers refer to CNC programs as
tapes. It is carried over from previous technology where
programs were stored on punch tapes.
ACRONYMS
CNC: Computer Numeric Control – also known as a
programmable motion controller
GRAWDE: Gear Rib Automated Wing Drilling
Equipment
HAWDE: Horizontal Automated Wing Drilling Equipment

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